The present invention relates to gas or combustion turbine apparatus, gas turbine electric power plants and control systems and operating methods therefor.
Industrial gas turbines may have varied cycle, structural and aerodynamic designs for a wide variety of uses. For example, gas turbines may employ the simple, regenerative, steam injection or combined cycle in driving an electric generator to produce electric power. Further, in these varied uses the gas turbine may have one or more shafts and many other rotor, casing, support, and combustion system structural features which can vary relatively widely among differently designed units. They may be aviation jet engines adapted for industrial service as described, for example, in an ASME paper entitled "The Pratt and Whitney Aircraft Jet Powered 121MW Electrical Peaking Unit" presented at the New York Meeting in November-December, 1964.
Gas turbine electric power plants are usable in base load, mid-range load and peak load power system applications. Combined cycle plants are normally usable for the base or mid-range applications, while the power plant which employs a gas turbine only as a generator drive typically is highly useful for peak load generation because of its relatively low investment cost. Although the heat rate for gas turbines is relatively high in relation to steam turbines, the investment savings for peak load application typically offsets the higher fuel cost factor. Another economic advantage for gas turbines is that power generation capacity can be added in relatively small blocks such as 25MW or 50MW as needed for expected system growth, thereby avoiding excessive capital expenditure and excessive system reserve requirements. Further background on peaking generation can be obtained in articles such as "Peaking Generation," a Special Report of Electric Light and Power dated November, 1966.
Startup availability and low forced outage rates are particularly important for peak load power plant applications of gas turbines. Thus, reliable gas turbine startup and standby operations are particularly important for power system security and reliability.
In the operation of gas turbine apparatus and electric power plants, various kinds of controls have been employed. Relay-pneumatic type systems form a large part of the prior art. More recently, electronic controls of the analog type have been employed as perhaps represented by U.S. Pat. No. 3,520,133 entitled "Gas Turbine Control System" and issued on July 14, 1970 to A. Loft, or by the control referred to in an article entitled "Speedtronic Control, Protection and Sequential System," and designated as GER-2461 in the General Electric Gas Turbine Reference Library. A wide variety of controls have been employed for aviation jet engines including electronic and computer controls as described, for example, in a March, 1968 ASME Paper presented by J. E. Bayati and R. M. Frazzini, and entitled "Digatec" (Digital Gas Turbine Engine Control), an April, 1967 paper in the Journal of the Royal Aeronautical Society authored by E. S. Eccles and entitled "The Use Of A Digital Computer For On-Line Control Of A Jet Engine," or a July, 1965 paper entitled "The Electronic Control Of Gas Turbine Engines" by A. Sadler, S. Tweedy and P. J. Colburn in the July, 1965 Journal of the Royal Aeronautical Society. However, the operational and control environment for jet engine operations differs considerably from that for industrial jet turbines. In referencing prior art publications or patents as background herein, no representation is made that the cited subject matter is the best prior art.
In connection with prior art gas turbine electric power plant operating and control systems and operating methods therefor, reference is made to copending related application Ser. No. 082,470 which, in conjunction with other enumerated related patent applications, comprises a description of an improved gas turbine plant operating and control system. The present disclosure represents a further advancement over the prior art discussion herein contained and should be considered as exclusive of the referenced application.
Generally, the operation of the industrial gas turbine apparatus and gas turbine power plants has been limited in flexibility, response speed, accuracy and reliability. Further limits have existed in the efficiency or economy with which single or multiple units are placed under operational control and management. Control loop arrangements and control system embodiments of such arrangements for industrial gas turbines have been less effective in operations control than is desirable. Limits have also existed on how close industrial gas turbines can operate to the turbine design limits over various speed and/or load ranges.
More particularly, in gas turbine control, substantially continuous monitoring of turbine parameters accurately reflecting operating conditions at the various operation cycle positions is essential. Optimum operation over a wide range of operating conditions can be assured only by such monitoring and by reliable, accurate control loop response to variations in one or more of such parameters. Further, certain critical parameters must be continuously sensed in order to prevent damage to combustor elements, hot parts, rotor blades, etc., in the event of over-temperature or overload conditions.
Process sensors of various types have been employed to furnish control system inputs. For example, temperature and pressure sensors have been located at various turbine cycle positions and in varying configurations.
Accurate reliable temperature and pressure indications have been increasingly recognized as essential to maintaining the integrity of a system having one or more control loops wherein it is sought to control turbine speed or load in response to a temperature and/or pressure derived fuel demand signal. During turbine startup, accurate combustor shell pressure indications have been found to be of particular importance. Again, under load accurate pressure readings may become essential to efficient operation.
During those modes of operation characterized principally by temperature control, the accuracy and reliability of such indications determine the degree to which optimum operating conditions may be attained. A description of an improved control system employing optimally arranged turbine system thermocouples, suitable for use in the gas turbine electric power plant of the present invention, may be found in copending application Ser. No. 155,905.
As gas turbine automatic control system developed, it became increasingly essential to obtain reliable temperature and pressure indications for use as control parameters in developing a fuel control input in the various control modes of operation. It became necessary to continuously review such measurements, not only for the purpose of assuring reliable, safe operation, but further to insure the availability of control variables which would enable efficient operation of the gas turbine near design limits to thereby enhance overall efficiency of the automatic control system. Known prior art control systems have lacked a facility for deriving consistently accurate control variables representative of critical parameters such as turbine inlet temperature, combustor shell pressure and turbine exhaust temperatures.
Although known prior art gas turbine control systems have provided multiple control loops in part responsive to temperature and pressure inputs, difficulties in obtaining continuous control over all operating modes has persisted, in a large part, as a result of an inability to obtain precise temperature and pressure inputs over a broad range of operating conditions. Over some portions of gas turbine operation, for example, temperature measuring errors and poor response of temperature measuring instrumentation have produced thermal lag so that response to step impulse inputs has been inadequate to achieving the highly responsive and flexible control necessary in most applications of gas turbine apparatus. Clearly, an alternative to temperature control has been indicated, in order to reduce undesirable thermal transients. Controlling as a function of combustor shell pressure over this interval of gas turbine operation presents an immediate alternative. However, problems have persisted in such control as hereinbefore indicated.
Problems encountered in controlling fuel system operation as a function of compressor shell pressure during gas turbine start-up and during subsequent modes of operation have indicated reliance on other operating parameters to achieve positive control during this time interval. Characteristically, such systems have not provided adequate control over a broad range of ambient temperatures. Variations in such ambient temperatures are known to cause significant variations in internal temperatures which may shorten the life of turbine components, such as blading and the like. A characteristic prior art control system calls for an initial shot of fuel upon detection of flame at light-off, with a subsequent cut-back from the initial impulse level to reduce thermal shock to hot path parts. At the end of the warm-up period, positive control is resumed as a function of temperature or acceleration. Clearly, such control is inadequate to preventing the effects of thermal transient or thermal shock to the critical turbine components.
Various methods and apparatus exist for obtaining, calibrating and displaying instantaneous values of critical turbine operating parameters. Characteristically, however, calibration of various instrumentation employed in obtaining control system inputs has been limited to a one-time setting prior to turbine start-up of instruments to indicate extreme values on a known scale, e.g., alignment of the particular dial at the zero and maximum setting, for example, a combustor shell pressure transducer provides readings from zero to 160 psig. Previously, calibration procedures had suggested an alignment of a dial at zero and 160 with an implicit assumption that increments between the two extreme values will be linearly a function of combustor shell pressure. Nowhere is there suggested in known prior art control systems a facility for dynamically correcting for transducer error. Prior art controllers have had no facility for remembering the zero point as read when the unit was shut down so that re-zeroing might be accomplished during gas turbine operation. For the foregoing and for other reasons, difficulties have existed in obtaining the reliable, accurate combustor shell pressure indications necessary to the provision of responsive surge control during gas turbine start-up and during the other operating modes of industrial gas turbine apparatus. Dynamic calibration techniques have been lacking so that calibration before start-up has been relied upon exclusively. Specifically a variety of field experiences have demonstrated particular problems in calibrating combustor shell pressure transducers so that they repeat exactly to a zero reading after shut-down. The readings in the vicinity of 0 pounds are very critical during the initial light-off period since, as discussed previously, combustor shell pressure is desirably considered in preventing compressor surge during this period. Variations in zero setting cause greatly varying light-off temperature control as verified by recorder traces taken in the field. Certain, otherwise adequate control algorithms and systems dictate inhibition of start-up if the pressure transducer is uncalibrated by more than one half pound at the zero point. Thus the problem of transducers repeating to zero pounds has affected availability and reliability.